dispatch: handle empty `testedwith` value in extension

When extensions had an empty `testedwith` attribute the code tried to parse it
and failed. As a result the actual error were shallowed by a This crash.

We now treat empty strip as 'unknown'

mercurial/dispatch.py

@@ -217,11 +217,11 b' def _runcatch(req):'
         ct = tuplever(compare)
         worst = None, ct, ''
         for name, mod in extensions.extensions():
-            testedwith = getattr(mod, 'testedwith', 'unknown')
+            testedwith = getattr(mod, 'testedwith', '')
             report = getattr(mod, 'buglink', _('the extension author.'))
-            if testedwith == 'unknown':
+            if not testedwith.strip():
                 # We found an untested extension. It's likely the culprit.
-                worst = name, testedwith, report
+                worst = name, 'unknown', report
                 break
             if compare not in testedwith.split() and testedwith != 'internal':
                 tested = [tuplever(v) for v in testedwith.split()]

tests/test-extension.t

@@ -498,6 +498,16 b' No declared supported version, extension'
   ** Python * (glob)
   ** Mercurial Distributed SCM * (glob)
   ** Extensions loaded: throw
+empty declaration of supported version, extension complains:
+  $ echo "testedwith = ''" >> throw.py
+  $ hg --config extensions.throw=throw.py throw 2>&1 | egrep '^\*\*'
+  ** Unknown exception encountered with possibly-broken third-party extension throw
+  ** which supports versions unknown of Mercurial.
+  ** Please disable throw and try your action again.
+  ** If that fixes the bug please report it to the extension author.
+  ** Python * (glob)
+  ** Mercurial Distributed SCM (*) (glob)
+  ** Extensions loaded: throw
 If the extension specifies a buglink, show that:
   $ echo 'buglink = "http://example.com/bts"' >> throw.py
   $ rm -f throw.pyc throw.pyo